Method for burning refining residues

ABSTRACT

The method of the invention for combustion of residues from refining petroleum in a boiler comprising a hearth and injectors is characterized in that cooled gas containing oxygen is injected into the hearth in the hottest region of said hearth. Injection into the very hot region of the hearth cracks tar or aromatic polycyclic hydrocarbons and converts them into basic compounds that are easily combustible and therefore less pollutant.

The present invention relates to a method and a device for burning residues from refining petroleum with minimum emission of nitrogen oxides and dust.

These residues, including tar, asphalt and bitumen, are of low commercial value and include in particular products that are solid at room temperature and have a viscosity in excess of 500 centistokes at 100° C. They are usually burned by refineries for their own purposes. The production of light diesel oil for automobiles in Europe leads to the production of these residues, for example.

Their combustion in conventional boilers is harmful to the environment, and new standards on the emission of oxides of sulfur in Europe, and in France in particular, make this solution impossible. These restrictions will also apply to dust, polycyclic aromatic hydrocarbons and volatile organic compounds.

Instead of burning these residues, it is possible to use other forms of treatment that are less harmful to the environment but more complex and more costly. Thus gasification may be used, either integrated into a combined cycle or taking the form of a deep conversion process that converts the residues to an economically viable final residue such as petroleum coke, for example.

The increased production of so-called “bottom of the barrel” residues (tar), in particular in Europe, which at present are burned by refineries in conventional boilers, will also lead to modification of the treatment of these residues.

The object of the present invention is to propose a method for the combustion of the residues previously cited and an injection device, both of which may be used in existing installations, in particular in tangential heating boilers, and which significantly reduce the emission of pollutants, primarily oxides of nitrogen and unburned carbon dust.

The method according to the invention for the combustion of residues from the refining of petroleum in a boiler comprising a hearth and injectors is characterized in that gas containing oxygen is injected into the hottest region of said hearth which, situated in the immediate vicinity of the burner, is one of cyclonic flow, i.e. one in which the combustion gases are caused to rotate about a vertical axis to ensure that they are totally converted, leaving no unburned substances. Injection into the very hot region of the hearth ensures high-temperature cracking of the residues and their conversion into basic compounds (CO, H₂, CH₄, CO₂) that are easily combustible and therefore less pollutant, because of the reduction of unburned carbon dust. The method applies in particular to tangential heating boilers.

According to another feature, recycled flue gases are simultaneously injected into the hottest region of the hearth. Simultaneous injection of recycled flue gases imparts movement to the jet of gas containing oxygen encouraging penetration of said gas into the hearth and in particular into the cyclonic hottest region. Injecting recycled flue gases also facilitates the homogenization of the combustion products resulting from cracking of the residues in the hearth, and thereby prevents temperature peaks in the flue gases in the vicinity of the heat exchangers disposed over the combustion region. Increased production of oxides of nitrogen (NOx) in the hot region is compensated by the low production of nitrogen oxide resulting from staged combustion using either staged air or staged fuel.

According to one particular feature, injection is effected by injectors disposed in the corners of the hearth. The injection position will be determined as a function of the nature of the residue to be burned. One or more injection levels may be provided as a function of the peak temperature in the hearth facing the barge-shaped arrangement of the fuel injector and as a function of the expected reduction in nitrogen oxides resulting from the staged combustion created in this way by combining injection of air and fuel and injection of recycled flue gases, oxygen and residue.

According to a second disposition, injection is effected by injectors on the lateral walls of the hearth.

According to one particular feature of the second disposition, the injectors are at the middle of each of the lateral walls of the hearth.

In a variant of the second disposition, the injectors are on the four walls.

In another variant of the second disposition, the injectors are on two facing walls.

The device in accordance with the invention for injecting gas into a boiler hearth is characterized in that it is installed in flue gas recirculation trunking and comprises concentric gas injection tubes. Combining two injections creates a hot conversion region. These devices may be installed at several locations in the hearth of the boiler.

According to one particular feature, the device is substantially perpendicular to the walls of the hearth in a horizontal plane. The location, inclination and deviation in the horizontal plane of the device are adapted as a function of the architecture of the boiler to encourage penetration into the hot combustion vortex region. The recirculated flue gas feed trunking may therefore be bent, which induces additional, substantially horizontal rotation of the incoming flow, and therefore creates or increases rotary flow about a substantially vertical axis of the flue gases recirculated into the hearth.

According to another particular feature, the device is substantially inclined vertically towards the bottom of the hearth. The angle of inclination is preferably less than 45°.

According to a second particular feature, the device comprises a cooling circuit. The temperature at the tip of the injection device is such that it is necessary to cool it.

According to a third feature, the device is swept by a gas to prevent it from becoming blocked by dust coming from the hearth.

According to a fourth feature, the device is retractable. It is therefore possible to carry out maintenance with the boiler in operation.

According to a fifth feature, the device comprises a gas pilot light which encourages self-ignition of the residue.

The invention will be better understood from the following description, which is given by way of example only and with reference to the appended drawings, in which:

FIG. 1 is a view in vertical section of a boiler according to the invention,

FIG. 2 is a view in section of the boiler according to the invention,

FIG. 3 is a detailed view in section of an injector according to the invention in an injection position,

FIG. 4 is a detailed view in section of the FIG. 3 injector in a retracted position, and

FIG. 5 is a detailed view in section of the injector pipe.

In FIG. 1, the boiler 1 comprises a hearth 2 to which is fed fuel 3 surrounded by hot air 4. Injectors 5 inject oxygen or gas containing oxygen 50, the residue 51, and (where applicable) recycled flue gases 52. A cooling liquid 53 such as water flows in the injectors 5 to cool them. The injectors 5 are placed at the height of the hottest region 20 of the hearth 2. Additional air is injected via the injector 6 to bring about staged combustion, in order to reduce the emission of oxides of nitrogen, the injectors 6 are located above the injectors 5 so that the flue gases remain between the two injection levels 5 and 6 for a period from 0.2 second to 5 seconds.

FIG. 2 shows the FIG. 1 boiler 1 from above, at the height of the hot region 20. The fuel 3 and the hot air 4 are injected in the corners of the boiler 1. In this variant the injectors 5 are placed at the middle of the lateral walls 21 of the hearth 2, but they could be placed in the corners. The injection of the fuel 3 in the corners creates a vertical axis vortex 200 to which the residue 51 and the gas 50 are steered.

The injector 5 shown in detail in FIG. 3 comprises a feed pipe 500 for oxygen (or a gas containing oxygen) 50, a pipe 510 for the residue 51, a feed pipe 530 for the cooling liquid 53 and a return pipe 531 for said liquid 53. The recycled flue gases 52 arrive via a pipe 520. The injector 5 is inclined downward at an angle of less than 45° to encourage penetration of the residue into the hot region 20. Below a region 22 for gasification of the residue 51 is a combustion region 23 and above it is a post-combustion region 24.

The injection pipe 54 discharging into the hearth 2 is shown in FIG. 5 and is made up of four concentric cylinders; the residue 51 flows in the central cylinder 540, the gas 50 enriched with oxygen flows between the cylinder 540 and the next cylinder 541, and the cooling liquid 53 flows between the cylinders 541, 542 and 543, making a return trip in the pipe 54 with the outward flow between the outermost cylinders 542 and 543 to facilitate cooling and the return flow between the innermost cylinders 541 and 542.

The flue gases 52 arrive via the pipe 520 and enter the pipe 521 of the injector 5 from which they are directed into the hearth 2 (see FIG. 3).

FIG. 4 shows the injector 5 in a retracted position, the assembly comprising the pipe 54 and the pipes 530, 531, 510 and 500 being retracted into the feed pipe 521 for the flue gases 52 so that this assembly of the injector 5 may be maintained in service.

The execution of the method is described next. The fuel 3 and the hot air 4 are injected into the lower portion 23 of the hearth 2. Combustion develops and the region 20 above the burner 30 is the hottest region. The residues 51 and the gas 50 enriched with oxygen are injected into this region 20, in which the residues 51 are cracked and reduced to combustible basic compounds. Simultaneous concentric injection of recycled flue gases 52 via the injectors 5 imparts movement to the jet of oxygen (or gas enriched with oxygen) 50 and to the residue 51 encouraging penetration of the residue 51 and the gas 50 into the vertical axis cyclonic flow region 200 of the hearth 2 and creates a hot conversion region encouraging the combustion of the residue 51.

The number of injectors 5 to be installed in the hearth 2 is decided as a function of the properties of the residue 51 to be burned. The injectors could be disposed at one or more levels as a function of the heat flow permitted by the walls 21 and the requirement to smoothe temperature peaks in the flue gases where they impinge on the exchanger bundles 7.

The inclination of the injectors 5 and their deviation in the vertical direction is less than 45°.

The injection system 5 comprises a gas pilot light to encourage self-ignition of the residue 51. 

1-14. (canceled)
 15. Method for combustion of residues from refining petroleum in a boiler having a hearth and injectors, said method comprising the steps of: injecting gas containing oxygen into the hearth in a hottest region of said hearth; simultaneously injecting recycled flue gases into the hottest region of the hearth.
 16. Method according to either claim 15, wherein injection is effected by injectors disposed in corners of the hearth.
 17. Method according to claim 15, wherein injection is effected by injectors disposed on lateral walls of the hearth.
 18. Method according to the preceding claim, wherein the injectors are placed at the middle of each lateral wall of the hearth.
 19. Method according to the preceding claim, wherein the injectors are placed on each of the four lateral walls.
 20. Method according to claim 18, wherein the injectors are placed on two facing walls.
 21. Device for injecting gas into a hearth of a tangential heating boiler for implementation of the process according to claim 1, wherein said device is installed in trunking for re-circulating flue gases and has concentric gas injection tubes.
 22. Device according to the preceding claim, wherein said device is disposed substantially perpendicular to the walls of the hearth in a horizontal plane.
 23. Device according to the preceding claim, wherein said device is substantially inclined vertically toward the bottom of the hearth.
 24. Device according to claim 21, wherein said device has a cooling circuit.
 25. Device according to claim 21, wherein said device is swept by gases.
 26. Device according to claim 21, wherein said device is retractable.
 27. Device according to claim 21, wherein said device has a gas pilot light. 